feat: agent uses Tailnet v2 API for DERPMap updates (#11698)

Switches the Agent to use Tailnet v2 API to get DERPMap updates.

Subsequent PRs will do the same for the CLI (`codersdk`) and `wsproxy`.

coderd/tailnet_test.go

@@ -178,6 +178,7 @@ func setupAgent(t *testing.T, agentAddresses []netip.Prefix) (uuid.UUID, agent.A
 	})
 
 	c := agenttest.NewClient(t, logger, manifest.AgentID, manifest, make(chan *agentsdk.Stats, 50), coord)
+	t.Cleanup(c.Close)
 
 	options := agent.Options{
 		Client:     c,